A reticle or photomask is a precision glass plate containing circuit layouts for use in a semiconductor fabrication process. If any dust or contaminant particles become attached to the reticle, such particles may result in artifacts on the printed circuits. A pellicle is a thin piece of transparent material mounted at a distance from the mask (reticle). The distance is chosen so that any dust or particles on the pellicle are out of focus when the wafer is in focus. Thus, any foreign particles that have fallen on the surface of the pellicle are not projected on the wafer, and do not contribute to printing defects.
Even with the use of a pellicle, other factors contribute to printing defects. One of these problems is reticle haze. It is well-known that haze is caused by the chemicals, SOx & NHx (e.g., SO2 and NH3) which are generated during the mask making process, and react with moisture. These chemicals may result from outgassing of materials used in making the masks, wet chemicals including sulphuric acid and hydrogen peroxide, and/or impurities from handling the masks. When these chemicals and moisture are present and exposed to ultraviolet (UV) light (such as the light used to expose a pattern onto a wafer), ammonium sulfate crystals grow, causing reticle haze. Thus, the life of the reticle can be shortened by formation of reticle haze crystals, which increase each time the mask is exposed to DUV (deep UV) light.
In order to retard or prevent the formation of haze, foundries have stored masks in controlled environments (e.g., storage in an inert gas) and employed “purge” operations to remove contaminants. A purge operation typically places the mask in an environment of pure nitrogen or extreme clean dry air (XCDA). Both of these environments can remove moisture and ammonia reactants from the mask, to avoid haze formation. For example, a purge operation may be performed immediately before and immediately after using the mask in a scanner.
U.S. Pat No. 6,593,034 describes a pellicle frame having two gas passage openings. The pellicle frame is adapted for replacing the air in the space between the mask and the pellicle with an inert gas, such as nitrogen, helium, argon or the like. The frame has a pair of openings, for feeding the inert gas into the space and for exhausting the gas from the space. The openings have filters which prevent subsequent infiltration of large particles. This configuration eliminates haze at the time the pellicle is mounted, but haze can still form subsequently during use.
Thus, improved methods of preventing reticle haze are desired.